1. Field of the Invention
The present invention relates to a high-frequency amplifier for amplifying the high-frequency signal used in, for example, a mobile communication terminal, and more particularly to a high-frequency amplification device equipped with an input/output matching circuit.
2. Description of the Related Art
High-frequency amplifiers are used to amplify the high-frequency signal transmitted from, for example, a mobile communication terminal. An input matching circuit and output matching circuit are connected to each high-frequency amplifier for matching the input/output impedance with that of an external circuit.
Size reduction of, for example, mobile communication terminals is now being developed. In accordance with this, there is also a demand for size reduction of high-frequency amplifiers and matching circuits of a large circuit scale for use in mobile communication terminals. To meet the demand, a technique for forming a matching circuit on a semiconductor chip has been proposed (see Jpn. Pat. Appln. KOKAI Publication No. 8-88523). In this technique, an increase in circuit scale due to the existence of chip components is suppressed by forming, in and on a semiconductor chip incorporating a high-frequency amplifier, all matching circuits that were so far provided on a dielectric substrate.
A GaAs substrate, for example, is used as a semiconductor substrate for processing a high-frequency signal. In the above prior art, however, the semiconductor chip must have a large area for a transmission line serving as an inductor component, inevitably resulting in an increase in the cost of the semiconductor chip. GaAs substrates are more expensive than Si substrates. Further, when a device having a hetero-junction structure, such as a hetero-junction bipolar transistor (HBT), is formed on a GaAs substrate, the GaAs substrate becomes even more expensive since the substrate must be subjected to an epitaxial growth process. Therefore, a transmission line that requires a large area on a semiconductor chip is an unignorable cost-increasing factor.
Further, the wiring resistance of a transmission line on a semiconductor chip is higher than that of a transmission line formed on a dielectric substrate. This is partially because the width of the transmission line is reduced to the order of several tens of μm to prevent an increase in chip area. Furthermore, since the transmission line is formed by a semiconductor process, its thickness is also about several μm, which is another factor of increasing the wiring resistance.
On the other hand, when a transmission line is formed on a dielectric substrate, it has a width of 100 μm or more and a thickness of at least several tens of μm. Thus, the wiring resistance is much lower than in the case of a transmission line formed on a semiconductor chip. A high wiring resistance causes a power loss in a matching circuit, which degrades the efficiency of high-frequency amplifiers.